Recently, composite films, which are produced by laminating component materials, including plastic films, metal foils such as aluminum foils, and barrier films such as metal deposited films and silica deposited films, by use of adhesive, has been widely used for packaging materials used in various industrial fields, including packaging materials for food products, beverages, medical products and quasi-drugs.
As laminating adhesives used for production of these composite films, two-part curing urethane adhesives made of a main component including polyester polyol or polyether polyol and a curing agent including organic polyisocyanate compound are widely used from a viewpoint that two-part curing urethane adhesives have excellent adhesion performance.
In recent years, in view of the importance of hygiene control, packaging bags made using these composite films are generally subjected to high-temperature sterilization treatment such as hot-water spray sterilization treatment, hot-water rotary sterilization treatment, or steam rotary sterilization treatment. However, there are some disadvantages such that, in composite films including metal foils and barrier films, particularly including metal foils, the plastic film and the metal foil in the composite film are peeled off from each other after the high-temperature sterilization treatment; when, as contents, acid food products such as vinegar, sauce and soy sauce, oily food products, or a mixture thereof are packed and then subjected to high-temperature sterilization treatment, the adhesive strength between the metal foil and the plastic film is remarkably reduced after the high-temperature sterilization treatment, or the metal foil and the plastic film are peeled off from each other.
As a method for overcoming the above disadvantages, a method for partially carboxylating terminal hydroxyl groups of the polyester polyol contained in the main component is known.
There has been proposed, for example, a dry laminating adhesive including a polyester composition having a carboxyl group at its molecular terminal and an isocyanate compound, in which the polyester composition includes polyester (A) having two or more hydroxyl groups at the terminal and having a molecular weight of 5000 to 10000, and polyester (B) obtained by carboxylating at least one of the terminal hydroxyl groups (cf. Patent Document 1, for example).
In addition, for example, an adhesive composition containing a mixture of 10 to 90% by weight of polyester polyurethane polyol and 10 to 90% by weight of polyester resin having a carboxyl group at its molecular terminal, orthophosphoric acid or its ester compound, and an organic isocyanate compound has been proposed (cf. Patent Document 2, for example).    Patent Document 1: Japanese Examined Patent Publication No. 03-68916    Patent Document 2: Japanese Unexamined Patent Publication No. 05-51574